A steel sheet for an automobile frame component is required to be increased in strength in order to achieve collision safety and improvement in fuel efficiency. The steel sheet is therefore required to have certain press formability while having higher strength of 980 MPa class or higher. As well known, steel produced using a TRIP effect is effectively used to achieve high strength and excellent formability of the high-strength steel sheet of 980 MPa class or higher (for example, see PTL1).
PTL1 discloses a high-strength steel sheet that contains bainite or bainitic ferrite as a main phase and 3% or more by area ratio of retained austenite (γR). The high-strength steel sheet, however, has a total elongation of less than 20% at a tensile strength of 980 MPa or more at room temperature, and is therefore required to be further improved in mechanical properties (hereinafter, also simply referred to as “properties”).
Moreover, although the TRIP steel sheet has excellent formability, a forming load of press working inevitably increases in correspondence to such high strength. Use of the TRIP steel sheet is therefore difficult depending on component size.
There has been proposed a technique referred to as hot press (or hot stamping) as a technique that decreases the forming load of press working, in which a steel sheet is pressed in a high temperature range of about 900° C. so that a forming load of press working is decreased, and is then controllably cooled to have a martensite microstructure, thereby achieving high strength (for example, see PTL2). However, this technique has disadvantages in manufacturing, such as extreme oxidization of a steel sheet during heating, long heating time, and indispensable cooling control. Hence, there has been required development of a technique that achieves reduction in forming load of press working and increase in strength in a relatively low temperature range.